In recent years the adhesive bonding of metal surfaces has become an important industrial practice in the assembly of articles of intricate shapes, which include jewelry and aircraft fuselage and wind components.
A recent discovery in this field is a group of adhesives which remain fluid or spreadable for useful periods of time when stored with access of oxygen (or air), but which set or "cure" rapidly when allowed to stand in the absence of air (i.e., in the absence of "free" or molecular oxygen). Such compositions are termed "anaerobically setting adhesives". They consist essentially of a liquid or spreadable polymerizable vinyl monomer (or a mixture of such monomers) and an organic peroxide polymerization initiator therefor, which remains inactive or ineffective in the presence of free oxygen but which rapidly initiates polymerization upon exclusion of oxygen, after which the polymerization goes to completion with formation of a strong adhesive bond.
A wide variety of vinyl monomers been found suitable for the purpose; the term "monomers" includes complex acrylic compounds which contain a plurality of vinyl substituents as well as other substituents. In the adhesives field, acrylic compounds are generally regarded as monomeric when they are of fluid or syrupy (paste) viscosity.
Acrylic compounds which are suitable for use in anaerobically setting adhesives in general are the polymerizable esters of two or mols of (meth) acrylic acid with appropriate alcohols, for example tetraethylene glycol diacrylate, tetraethylene glycol di (chloroacrylate), diglyceryl diacrylate, diglycerol tetramethacrylate, neopentyl glycol diacrylate, and trimethylol propane triacrylate.
Suitable esters can also be formed by reacting an acrylate ester containing a reactive hydroxyl or amine group in the non-acrylate portion with an organic disocyanate, and a wide variety of such esters is known. Diisocyanates suitable for the above purpose include toluene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, and cyclohexylene diisocyanate.
Compositions of the foregoing type are disclosed in Gruber et al. U.S. applications Ser. Nos. 607,463; 607,465; 607,466; and 607,467 now respectively U.S. Pat. Nos. and in U.S. Pat. 2,626,178; 2,895,950; 3,041,322; 3,043,820; 3,046,262; 3,218,305; 3,425,988; 3,625,930; and 3,682,875.
In general, the diacrylic and dimethacrylic esters of dihydroxy alcohols are suitable for use in the manufacture of liquid or paste mixtures which polymerize in the absence of air and which are suitable for adhesive purposes. The di-, tri- and tetraethylene glycols ester are particularly suitable.
Furthermore, the acrylic and methacrylic esters of monohydroxy alcohols such as cyclopentanol, methylcyclopentanol, cyclohexanol, methylcyclohexanol, methylcyclopentane, methylolcyclohexane, borneol, tetrahydrofurfuryl alcohol and 1,3-dioxa-2,2-dimethyl-4-methylol cyclopentane have been used for these purposes.
However, the known adhesives frequently possess certain disadvantages, particularly the disadvantage of unsatisfactory adhesion to metal surfaces. Metal surfaces to be adhered to one another with these adhesives usually have to be subjected to special pretreatment with primers such as isocyanates or the like.
According to U.S. Pat. No. 3,300,547, the adhesiveness of such compositions for steel is enhanced by the action of a polymerizable terminal double bond for ethylene linkage adjacent to a carboxyl substituent. However, carboxyl substituents have a corrosive action towards steel.